Surface structures of the two allotropic phases of cobalt

The atomic surface structures of the close-packed faces of single crystalline cobalt in the bulk hcp and bulk fcc phases have been determined by low-energy electron diffraction. The two separate structures were monitored on a single-crystal sample cycled through the surface-transformation temperature of 450 \ifmmode\pm\else\textpm\fi{} 10\ifmmode^\circ\else\textdegree\fi{}C. The structure of the low-temperature phase was determined to be an hcp (0001) surface layer on an hcp bulk crystal with a top surface-layer spacing equivalent to that of the bulk 2.05 \AA{}. The surface structure of the high-temperature phase was determined to be an fcc (111) surface layer on an fcc bulk crystal with a top surface-layer spacing equivalent to that of the bulk 2.05 \AA{}.